This invention relates to optical fibers, and, more particularly, to multi-core optical fibers and methods for manufacturing the same.
With the expansion of telecommunications services, there has been a great demand for bandwidth on optical fibers. The information capacity of optical fibers can typically be enhanced by increasing the number of channels per fiber through wavelength division multiplexing (WDM) and/or by increasing bit rate using time-division multiplexing (TDM). This increase is limited eventually by the amount of information that can be carried on one fiber. The efficiency with which information is transmitted through a fiber is generally given by the following metric:
Efficiency=Bit Rate/channel spacing*100xe2x80x83xe2x80x83(1)
where channel spacing is the separation between the nearest channels. Thus, an efficiency of 100% is achieved when bit rate is equal to the channel spacing. Generally, for currently available return to zero (RZ) and non-return to zero (NRZ), systems the proven efficiency is at about 40% although a spectral efficiency as high as 80% was reported in xe2x80x9cTransmission Over 3xc3x97100 Km of Terralight(trademark) Fiber,xe2x80x9d Bigo et al., PD 1-2, European Conference on Optical Communications (ECOC) 2000, Germany. The level of spectral efficiency that will be achieved will, typically, eventually be defined by single channel non-linearities and/or cross channel non-linearities.
An alternate way to increase the capacity of optical fibers is to increase the number of fiber counts. From this standpoint it is generally desirable to pack as many fibers as possible per unit area. This approach may be limited by a variety of factors. For example, the outer cladding plays a significant role although generally only in the case of fibers that are very weakly guided. The outer secondary coating typically protects the glass optical fiber and the fundamental mode from perturbations that are introduced during the cabling process. By reducing the cladding diameter and by getting many cores to share a single secondary coating it would appear to be possible to increase the packing density of cores per unit area in a cable. However, a problem is encountered if cores are too close as light can couple from one core to another. This cross-talk limits the ability to pack more cores in a fiber.
Multi-core optical fibers are provided in various embodiments of the present invention which include a plurality of cores. At least two cores of the plurality of cores have different associated mean propagation constants at a reference wavelength. The difference between the associated mean propagation constants may be selected to reduce cross-talk between the at least two cores as compared to cores having a same associated mean propagation constant. A primary coating of light absorbing material may be positioned between the cores to further reduce cross-talk. Also provided are methods for manufacturing the multi-core optical fibers of the present invention and wavelength division multiplexing systems using the same.
In further embodiments of the present invention, the difference between the associated mean propagation constants is selected to provide a difference between propagation constants of locally adjacent portions of the at least two cores of at least about 0.005% throughout a length of the fiber. The difference between the associated mean propagation constants may be at least about 0.05%. The separation between adjacent cores may be less than about 70 microns. The cross-talk between the cores may be below about xe2x88x9240 decibels (dB) at the reference wavelength and, in further embodiments, may be less than about xe2x88x9230 dB at the reference wavelength.
In other embodiments of the present invention, a primary coating is provided between the cores that reduces the cross-talk between the at least two cores. The cores may be a material selected from a group consisting of pure silica and silica doped to modify its index of refraction. The fiber also includes a cladding layer around the cores. The primary coating in various embodiments is between the cladding and ones of the cores. The cladding layer may be a material selected from a group consisting of pure silica and silica doped to modify its index of refraction. A secondary coating may be provided around the cladding layer.
In further embodiments of the present invention, the fiber has two cores. The two cores may each have a diameter of between about 5 and about 15 microns. The cladding layer may have a diameter of between about 120 and about 150 microns. The separation between the two cores may be less than about 70 microns. In various embodiments, the difference between the associated mean propagation constants of the two cores is provided by either a difference between diameters of the two cores or a difference between core deltas of the two cores.
In other embodiments of the present invention, the multi-core fiber includes seven cores. Adjacent ones of the seven cores have different associated propagation constants. The cores may each have a diameter of between about 4 and about 15 microns with a core separation of about 70 to about 125 microns or, in further embodiments, a core separation of less than or equal to about 70 microns. The cross-talk between adjacent cores in various embodiments may be less than about xe2x88x9230 decibels (dB) and may further be less than about xe2x88x9240 dB. The fiber may be an all glass cladding multi-core fiber or a hybrid glass/polymer cladding multi-core fiber.
In a further aspect of the present invention, methods are provided for manufacturing a multi-core fiber. A plurality of canes including a core material and a cladding material around the core material are formed. The plurality of canes are coupled to provide a preform. The multi-core fiber is drawn from the preform to provide a multi-core fiber having a plurality of cores having different associated propagation constants. The different propagation constants are provided by either a difference between diameters of at least adjacent ones of the plurality of cores or a difference between core deltas of at least adjacent ones of the plurality of cores.
The canes may be formed using an outside vapor deposition (OVD) process. A secondary coating may be provided around the drawn multi-core fiber. A primary coating of a lossy material may also be provided between the core material and the cladding material so that the drawn fiber includes a primary coating layer between ones of the plurality of cores.
In other embodiments of the present invention, the multi-core fiber is an all glass fiber. A plurality of bait rods including a core material are formed. The formed bait rods are positioned in a multi-chuck lathe in relative locations selected based on a desired distance between cores of the multi-core fiber. An overcladding layer is formed around the positioned bait rods to provide the preform. A central bait rod may be positioned in the multi-chuck lathe inside the positioned bait rods. The multi-core fiber may be a seven core fiber in which case the central bait rod may include a core material and a cladding material. In other embodiments, the central bait rod is a light absorbing glass. Forming an overcladding layer around the positioned bait rods may include concurrently oversooting the positioned bait rods and the central bait rod.
In further embodiments of the present invention, methods are provided for manufacturing a multi-core fiber. A plurality of canes including a core material and a cladding material around the core material are formed. Each of the plurality of canes is separately drawn to provide a plurality of single core fiber elements each including a core and a cladding layer, the cores having different associated propagation constants. The different propagation constants may be provided by either a difference between diameters of adjacent ones of the cores or a difference between core deltas of adjacent ones of the cores. The plurality of single core fiber elements are coupled to provide a multi-core fiber having a plurality of cores. The canes may be formed using outside vapor deposition (OVD).
A secondary coating may provided around the plurality of single core fiber elements. Also, a primary coating of a lossy material may be formed between the core material and the cladding material of each of the canes to provide the single core fiber elements with a primary coating layer between their core and their cladding layer.
In other embodiments of the present invention, the multi-core fiber is a hybrid fiber, In such embodiments, coupling the plurality of single core fiber elements to provide a multi-core fiber having a plurality of cores includes assembling the plurality of single core fiber elements in a desired geometry. A polymer filler is added to the assembled plurality of single core fiber elements. The single core fiber elements and polymer filler are extruded together to position polymer filler between ones of the single core fiber elements. A secondary coating may be provided around the extruded single core fiber elements and polymer filler.
In a further aspect of the present invention, wavelength division multiplexing (WDM) systems are provided. An optical input device receives data for transmission from one or more data sources. A multi-core optical fiber is coupled to the input device. The multi-core optical fiber has a first core that has a first associated mean propagation constant and a second core that has a second associated mean propagation constant different from the first mean propagation constant. An optical output device is provided that is configured to couple the multi-core optical fiber to a destination device.
The first core may define a first communication path between the input device and the output device and the second core may define a second communication path between the input device and the output device. Each communication path may have an associated plurality of transmission wavelengths and alternate ones of an available set of wavelengths for WDM communications may be provided to respective ones of the communication paths so as to increase a wavelength difference between adjacent transmission wavelengths of each of the associated plurality of transmission wavelengths. The transmission wavelength allocations may be provided while maintaining the number of transmission wavelengths supported between the input device and the output device.
The optical input device may be an optical input coupler. The optical output device may be an optical output coupler. A multi-port erbium-doped power amplifier (EDFA) may be coupled to the multi-core optical fiber by the output coupler.